1. Field of the Invention
This invention relates to laser shock peened articles and a method of laser shock peening, more particularly, for producing a laser shock peened ripstop array of non-overlapping spaced apart laser shock peened spots having localized compressive residual stresses imparted by laser shock peening.
2. Description of Related Art
Gas turbine engines and, in particular, aircraft gas turbine engines rotors operate at high rotational speeds that produce high tensile and vibratory stress fields within the blade and make the fan blades susceptible to foreign article damage (FOD). Vibrations may also be caused by vane wakes and inlet pressure distortions as well as other aerodynamic phenomena. This FOD causes nicks and tears and hence stress concentrations in leading and trailing edges of fan blade airfoils. These nicks and tears become the source of high stress concentrations or stress risers and severely limit the life of these blades due to High Cycle Fatigue (HCF) from vibratory stresses.
Therefore, it is highly desirable to design and construct longer lasting fan and compressor blades, as well as other hard metallic parts, that are better able to resist both low and high cycle fatigue and that can arrest cracks than present day parts. The above referenced U.S. Patent Applications are directed towards this end. They teach to provide an airfoil of a fan blade with a continuous or volumetric region of deep compressive residual stresses imparted by laser shock peening over at least an inwardly extending portion of laser shock peened surfaces of an article such as the fan blade. These regions are formed by multiple overlapping protrusions of compressive residual stresses imparted by laser shock peening that extend inward from overlapping. laser shock peened circles or spots.
The deep compressive residual stresses imparted by laser shock peening of the present invention is not to be confused with a surface layer zone of a work piece that contains locally bounded compressive residual stresses that are induced by a hardening operation using a laser beam to locally heat and thereby harden the work piece such as that which is disclosed in U.S. Pat. No. 5,235,838, entitled "Method and apparatus for truing or straightening out of true work pieces". The prior art teaches the use of multiple radiation pulses from high powered pulsed lasers and large laser spot diameters of about 1 cm to produce shock waves on the surface of a work piece similar like the above referenced patent applications and U.S. Pat. No. 3,850,698, entitled "Altering Material Properties"; U.S. Pat. No. 4,401,477, entitled "Laser shock processing"; and U.S. Pat. No. 5,131,957, entitled "Material Properties". Laser shock peening as understood in the art and as used herein, means utilizing a laser beam from a laser beam source to produce a continuous region of strong compressive residual stresses in a continuous region on a portion of a surface. The region is volumetric and produced by the coalescence of individual protrusions extending inward from overlapping laser shock peened circles or spots. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled "Laser Peening System and Method". Manufacturing costs of the laser shock peening process is a great area of concern because startup and operation costs can be very expensive. The "on the fly" laser shock peening process disclosed in Ser. No. 08/362,362, above is designed to provide cost saving methods for laser shock peening as is the present invention. Prior art teaches to use large laser spots, on the order of 1 cm and greater in diameter, and high powered lasers. Manufacturers are constantly seeking methods to reduce the time, cost, and complexity of such processes. A laser shock peening method that uses a low power laser beam, on the order of 3-10 joules, with a preferred range of 3-7 Joules and laser beam spots having a diameter of about 1 mm is disclosed in co-pending U.S. patent application Ser. No. 08/993,194 now U.S. Pat. No. 6,005,219, entitled "LASER SHOCK PEENING USING LOW ENERGY LASER" and this method is directed to reducing time, cost, and complexity of laser shock peening. There is an ever present desire to design techniques that result in such reductions and to this end the present invention is directed.